feat(#93): notification bell with upload progress

Replace the floating upload toast with a notification bell in the top bar
(between language selector and user avatar). All upload progress, completion,
and quota errors now flow through the bell dropdown panel.

- Add notification bell button with animated badge counter
- Dropdown panel shows per-file upload progress bars and overall batch progress
- Bell rings on new notifications when panel is closed
- Upload success/error states with color-coded icons
- Quota exceeded errors shown as notification items
- Clear all button to dismiss notifications
- Panel auto-opens when upload starts
- Full dark mode support
- Mutual exclusion with user menu (opening one closes the other)
- i18n keys for en/es (notifications.title, notifications.empty)
- SW cache bump to v10

Files:
- static/js/notifications.js (new module)
- static/index.html: bell markup + remove old toast
- static/css/style.css: bell + panel styles + dark mode
- static/js/fileOperations.js: redirect upload progress to notification bell
- static/js/app.js: close bell when user menu opens
- static/locales/{en,es}.json: i18n keys
- static/sw.js: cache v10 + notifications.js asset

static/sw.js

@@ -1,10 +1,11 @@
 // OxiCloud Service Worker
-const CACHE_NAME = 'oxicloud-cache-v9';
+const CACHE_NAME = 'oxicloud-cache-v10';
 const ASSETS_TO_CACHE = [
   '/',
   '/index.html',
   '/js/i18n.js',
   '/js/languageSelector.js',
+  '/js/notifications.js',
   '/locales/en.json',
   '/locales/es.json',
   '/locales/fa.json',